Switched mode power converters are typically constructed by mounting electronic components onto a printed circuit board (PCB). One electrical component typically included in a switched mode power converter is a transformer, which is one example of an energy transfer element. During operation the transformer allows the transfer of energy between an input side (referred to as a primary side) of the power converter and an output side (referred to as the secondary side) of the power converter. The input and output sides of the transformer are typically galvanically isolated from each other. Galvanic isolation occurs when dc current is unable to flow between the input side and output side of the power converter.
Transformers typically include coils of wire wound around a structure called a bobbin. The bobbin provides support for the coils of wire and also provides an area for a core of magnetically active material (such as ferrite or steel) to be inserted so that the coils of wire can encircle the core. The coils of wire make up the primary and secondary windings of the transformer. The core provides a path for a magnetic field generated by an electric current flowing through the coils of wire. The area around the bobbin where coils of wire can be wound is often referred to as the bobbin window.
Safety regulations require a minimum amount of creepage distance and clearance distance between the coils of wire of the primary, coils of wire of the secondary, and the core. Creepage distance is the length of the shortest path between two conductive parts along the surface of the structure separating them. Clearance distance is the shortest distance through the air between two conductive parts. To meet creepage and clearance distances, a designer may have to sacrifice usage of the bobbin window. In other words, some applications are unable to utilize the entire bobbin window while meeting creepage and clearance distances. For applications that utilize the entire bobbin window, the design of the bobbin (and the overall transformer) can be bulky, which is undesirable. Design of a bobbin is typically a compromise between copper loss, core loss, size and the safety considerations of creepage distance, clearance distance, and insulation requirements.